The present invention relates to a power steering system having an electric motor incorporated therein, and more particularly to a power steering system comprising an input shaft fixed to a steering wheel, an output shaft coupled to an electric motor via a reduction gear, and a torsion bar provided between the input and output shafts, such that the output shaft is rotatable by the resultant of a torque for the person to rotate the steering wheel and the torque of the motor.
In the specification, the torque for the person to rotate the steering wheel will be referred to as an "input torque," the torque of the motor as an "auxiliary torque," and the resultant torque as a "steering torque."
Such a power steering system is known which comprises means for detecting the input torque, and control means for for controlling the auxiliary torque, i.e. the current through the motor, in accordance with the output of the detecting means (see Unexamined Japanese Patent Publication No. SHO 59-77966). The conventional system involves a linear relation between the input torque and the motor current target value as shown in FIG. 6 and therefore has the following problem. The steering torque involved in stopping a motor vehicle differs greatly from that required during running, and stopping requires a greater torque than running. With the conventional motor-incorporating power steering system, however, the input torque has a linear relationship with the motor current target value, so that if a suitable magnitude of auxiliary torque is set for stopping, the auxiliary torque during running becomes excessive to make the steering wheel feel too light to handle, whereas when the steering toque is set to a suitable magnitude for running, too small a torque will result to make the wheel feel heavier when the vehicle is to be stopped. Accordingly, it has been proposed to control the steering torque in response to the speed of the vehicle, but this mode of control generally encounters difficulties in a very low speed range because of the limitations imposed on the speed sensor and signal detection circuit.